Master hologram for the production of copy-proof holograms

ABSTRACT

A method for the production of holograms proof against copying and imitation possessing authenticity features, which are copies of a master hologram. In accordance with the invention as a master hologram a hologram of a randomly structured optical element, as for example a diffusion plate, which constitutes the authenticity feature, is recorded and each hologram copy produced from the master hologram is provided with a layer having at least one layer with a color succession and/or the hologram copy produced on a thick light sensitive layer is caused to shrink or to expand.

This is a divisional of copending application Ser. No. 08/564,411 filedNov. 29, 1995.

BACKGROUND OF THE INVENTION

The invention relates to a method for the production of copy-proof andimitation-proof holograms possessing authenticity features, which arecopies of a master hologram.

Holograms, more particularly in the form of embossed holograms are beingincreasingly employed on identity documents and for vouching for theauthenticity of goods coming from reputed manufacturers. The desiredprotective function can however only be ensured by holograms, if theycan not be copied or imitated using non-holographic method by ingenious,resourceful forgers. It has been shown that presently conventionalholograms employed as proofs of authenticity can be copied.

SUMMARY OF THE INVENTION

One object of the invention is consequently to provide a method of thetype initially mentioned, using which copy-proof and imitation-proofholograms may be obtained capable of being employed as proofs ofauthenticity on articles and documents of all types.

In accordance with the invention such object is to be attained by thefeatures of a method for the production of copy-proof andimitation-proof holograms possessing authenticity features, which arecopies of a master hologram,

characterized in that

as a master hologram a hologram of a randomly structured opticalelement, as for example of a diffusion plate, which constitutes theauthenticity feature, is recorded and in that each hologram copyproduced from the master hologram is provided with a layer possessing atleast one succession of colors and/or the hologram copy recorded on athick light sensitive layer is caused to shrink or to expand.

Holograms suitable for use as a proof of authenticity must not only beproof against copying but also against imitation. Copy-proof here meansthat it is not to be possible for a forger to obtain an object, which isas similar as possible to the object recorded in the hologram serving asa proof of authenticity and then produce holograms from it, which thenrepresent the imitated holograms.

The optical element with a random structure encoded in the masterhologram, as for example a diffusing screen or a statistical phase-platewith a random distribution of the phase elements, may not be imitated byan artful forger, because to do this he would have to possess theoptical element, which is best destroyed after it has been employed forrecording the hologram, and which is however not available andfurthermore owing to its random structures can not be producedafterwards either.

In any event, owing to the use of the master hologram, which is aholographic recording of a randomly structured optical element,imitation by reproduction is prevented.

The necessary impossibility of copying is achieved by the securityhologram in accordance with the invention because the hologram copyproduced from the master hologram is provided with layer having at leastone succession of colors.

In lieu of using a layer having at least one layer with a succession ofcolors to render it copy-proof, the security of the security hologrammay also be achieved if the master hologram, produced on a thicklight-sensitive layer, is shrunk and/or caused to expand. Such shrinkageand/or expansion leads to color shifts in the hologram so that same mayno longer be copied monochromatically.

Rendering the security hologram copy-proof by a layer possessingsuccessions of colors and by shrinkage and/or expansion of the masterhologram may naturally also be performed cumulatively.

It is convenient if for improving security against forgery duringrecording the security hologram a distorted reference beam is employed.Starting with the security holograms produced from the master hologram,it is practically impossible for a forger to reconstruct the identicalmaster hologram, since for this purpose he would require the referencebeam utilized during recording of the security holograms, which forexample may be slightly distorted in a manner not able to be reproduced.

The security holograms in accordance with the invention may be recordedas reflection holograms or also as transmission holograms.

For the mass production of security holograms serving as a proof ofauthenticity an embossing stamp is obtained, for instance using knownmethods, from the master hologram and with same it is possible to embossthe security hologram, again using known methods, in aluminized or alsotransparent plastic layers.

Effective protection of such security holograms against copying ispossible if same are embossed in a film having a succession of colors.The succession of colors is preferably to be incorporated in theembossing lacquer layer. The successions of colors should be produced ina plurality of areas as stripes, dots or graphic effects in at least twocolors.

The term color successions is herein means stripes or patches indifferent colors, possibly produced in the form of control bars andpossibly with a variation of the colors within a stripe or patch in wavelength (color) or color density. As successions of colors it is possibleto provide graphic effects, dots or furthermore patterns responsible forspecial effects. Graphic effects or patterns producing special effectsmay be represented in the succession of colors. Another or additionalpossibility is one in which the colors change with the angle of viewingthe hologram, as for example is the case with colors of thin layers,which result from interference.

At least three colors should be present, complete transparency of thesubstrate layer counting as a color. If anything, a hologram renderedcopy-proof in this manner could only copied using very sophisticatedcopying methods with dye lasers corresponding to the colors, it hardlybeing possible, even with skillful copying tests, to avoid differencesin brightness in the copy; more particularly color boundaries wouldremain visible as lines. If the hologram were to be embossed on anidentical or similar film provided with a color pattern, the resultwould be a hologram having a brightness line pattern superimposed on thecolor pattern.

It is particularly convenient if aperiodic wave line patterns of colorlines are employed as colors patterns, wherein there is no lineintersection. During copying the line pattern would be transferred tothe copied product. On renewed embossing of the copy onto a substratefilm with the same or a similar wave line pattern the result would be ahologram with intersections between color and brightness lines whichcould serve a means of recognizing a fake (having a moire pattern). Amoire effect is produced, given a sufficiently fine line structure,owing to the superimposition of the different line structures of thefilm, color lines and hologram copy with the brightness lines, suchmoire effect rendering a fake easy to detect. By the same tokenso-called "guilloche" line patterns as on banknotes and sharecertificates can be used here.

The color structures may be designed in the form of pictures,photographs, graphic effects or a dot pattern with a random distributionor in the form of writing. The color dots do not have to be drawn withsharp boundaries and indeed they may possess a gradual color transition.

Such color successions can be applied with the aid of thermal embossingfilms. For this purpose an embossing lacquer is applied to a substratefilm using a separating adhesive layer. This embossing lacquer isprovided with a vapor-deposited aluminum layer. The hologram relief isthen embossed in such layer using the embossing stamp produced from themaster hologram. A hot melt adhesive layer (sizing/priming) is thenapplied to such embossed layer with the relief. Together with a printingstamp the compound so formed is pressed onto a substrate layer of paperor of plastic film. This substrate layer may for example be the documentto be validated i. e. whose authenticity is to be proved. Such pressingon is also performed while hot so that separation at the adhesiveseparation layer occurs. The priming layer then provides the connectionwith the substrate layer, the separating adhesive layer being releasedby the action of heat so that the embossing lacquer layer constitutesthe external layer.

The color successions may be already incorporated in the embossinglacquer layer beforehand. It is naturally also possible to apply afurther layer with the color succession. All layers may be transparent.

The hologram relief may also be embossed on a non-vapor coated plasticfilm, a reflection hologram being produced owing to the differentindices of refraction of the layers joined together.

As an alternative it is possible for an extremely thin aluminum layer tobe applied to a plastic film, which then partially remains transparent.

Mass production of thick holograms from master holograms may for exampletake place by contact copying with a scan exposure process in a knownmanner.

The identical holograms mass produced from the master hologram areprotected against copying by the embossing of the hologram intransparent films provided with different color areas or using aphotographic exposure method for the hologram and following shrinkageand expansion. However such protection will not in principle besufficient to avoid copying of the hologram by cunning forgers. For thisreason the hologram is also protected by coding, using the hologram of arandomly structured optical element, as for example a diffusion plate.

Since consequently the holograms mass produced from the master hologramincorporate encoded hologram information, special reading devices mustbe created with which such holograms may be recognized as beingauthentic. Such a reading device for examining the authenticity ofhologram copies produced from the master hologram is characterized inaccordance with the invention in that in a predetermined opticalarrangement with a laser, preferably a laser diode, a hologram of themaster hologram is produced using a reference beam, which may containreadable information, and in that the reading device has the very sameoptical arrangement as employed during recording, the hologram copy tobe examined being placed in the position of the master hologram. In thereading device the hologram of the master hologram consequentlyconstitutes the decoding hologram for examination of the authenticity ofthe hologram copies to be checked. If in the reading device the hologramto be checked is exposed using the reference beam also employed forrecording the decoding hologram, then from the hologram to be checked anobject beam will be reconstructed containing the hologram informationand which from the decoding hologram, if the hologram to be checked isauthentic, will reconstruct the reference beam employed duringrecording. This reference beam may be a continuous or even wave orhowever it may also contain readable information. Such reference beammay be automatically checked for authenticity by a light sensitiveoptical element.

The particular feature of the reading device in accordance with theinvention is that the master hologram or, respectively, the authenticholograms produced from it, can not be derived from the decodinghologram in the reading device. Since, even in the case of selection ofthe right reference, the decoding hologram will possess the tolerancesand unique features of the individually produced reading device, itwould be only be possible, if anything, using the reading device, toproduce and copy hologram which possesses the individual features of theunique reading device. Only if very great skill is employed could such ahologram copied from the decoding hologram produce a functioninghologram in the reading device, in which it has been copied, but not inother reading devices, which possess other unique departures due tomanufacture and tolerances.

Reconstruction of the master hologram from the decoding hologram isrendered extremely difficult or impossible not only because of thedistortion caused by the unique reading device, but also because anyforger would employ the read beam of the reading device as a referencebeam for re-copying.

In accordance with a further development of the invention there is aprovision such that on recording the decoding hologram "in situ" in thereading device the copy of the master hologram is produced using afourier lens in the fourier plane. The use of a known fourier opticalsystem offers the advantage that the image of the hologram is formed byparallel beams at infinity. This arrangement renders it possible, oninsertion of the hologram to be examined, to make adjustments in the Xand Y directions without the hologram reconstruction being adverselyaffected, since owing to high redundancy each dot, for reconstruction ofthe associated local frequencies, coincides with the correspondinggrating structures of the hologram and there is a reconstruction of thehologram. However the hologram to be checked must be inserted in thecorrect rotational setting. Such setting may be simply detected byslowly rotating the hologram during examination so that it will sometime arrive in the correct angular position, this being indicated by asudden increase in brightness of the reference beam indicatingauthenticity and reconstructed from the decoding hologram.

An additional way being certain that the master hologram may not bereconstructed from the decoding hologram of the reading device is byskewing the decoding hologram in each reading device by a differentangle.

BRIEF DESCRIPTION OF THE DRAWINGS

One embodiment of the invention will now be described in the followingwith reference to the accompanying drawing, in which

FIG. 1 shows an arrangement for recording a master hologram withhologram information encoded therein and consisting of a randomlystructured optical element in a diagrammatic elevation,

FIG. 2 shows an arrangement for the production of a reading device in adiagrammatic view,

FIGS. 3(a) and 3(b) are schematic illustrations of holograms possessinga succession of colors,

FIG. 4 is a schematic illustration of a master hologram directlyembossed into a plastic layer, and

FIG. 5 is a schematic illustration of production of a hologram copy froman embossing stamp.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 diagrammatically shows the recording of a master hologram,wherein the random structure of the diffusion plate 1 as an object isencoded. The diffusion plate 1 has an object beam 2, coming from alaser, shining through it. Using the image forming optical system 3,which comprises a known fourier optical system, the mutually parallelobject beams (for image formation at infinity) in the hologram plane 4are brought together with a reference beam 5 which is distorted orfurthermore which may be reproduced as required, to cause interferenceso that the master hologram is produced on the photographic material 6arranged in the hologram plane 4. Dependent on which side the referencebeam is incident on the hologram plane, a reflection hologram or atransmission hologram will be produced. If a thick hologram, which issubsequently to be caused to shrink or expand is to be produced, suchhologram is preferably produced in the form of a reflection hologramwith the reference beam 5.

Then using conventional mass copying methods holograms serving as aproof of authenticity are then made from the master hologram 6.

Using the master hologram or a copy of the master hologram the readingdevice is then produced with the aid of the arrangement illustrated inFIG. 2. Such reading device possesses a laser light source, preferably alaser diode, from which the reference beam 10 comes, with which ahologram is reconstructed from the master hologram 6 placed in thechecking plane 11, the reconstructed object wave 20 being projected bythe image forming optical system 12, which also comprises a fourieroptical system, in the hologram plane 13, in which plane thereconstructed object wave 20 is caused to interfere with a referencebeam 15 from the laser, such beam 15 possibly containing additionalinformation which is projected as image 16' using the image formingoptical system 17 as image 16. It is in this manner that in the hologramplane 13 a decoding hologram 14 is produced "in situ", which remains atthe same position in the unique reading device. The decoding hologram ishence not identical to the master hologram 6, since same has not beenreconstructed with the reference beam 15 but with a readingdevice-related beam 10 and furthermore represents the previouslytransformed object wave 20. If now in the reading device produced inthis manner the hologram to be examined is placed in the object plane11, 20 it is possible to reconstruct beam 15. In order to achieve thisreconstruction of beam 15, the hologram placed in plane 11 is irradiatedby reference beam 10. By interaction of reference beam 10 with thehologram in plate 11, object beam 20 is reconstructed and projected ontothe decoding hologram 14 via the optical system 12. By interaction ofthe object beam 20 with decoding hologram 14, the reference beam 15 willbe exactly reconstructed so that the information recorded therein willbe visible as a sign of authenticity and may also be machine-processed.

In FIG. 4, a master hologram 6 is directly embossed into a plastic layer26, whereby a hologram copy 6b is produced. In FIG. 5, an embossingstamp 6a is first produced from the hologram master in a known way.Then, embossing stamp 6a is embossed into a plastic layer 26, therebyproducing a hologram copy 6b, i.e., recording the hologram copy in theplastic layer.

We claim:
 1. A method for authenticity examination of a hologram copy(6) of a master hologram, comprising the following steps:arranging thehologram copy (6) in a first position (11) of the optical arrangement,arranging a decoding (reading) hologram (14) in a second position of theoptical arrangement, exposing the decoding hologram (14) to a firstreference (readout) beam (15) from an image-forming optical system (17),producing a readout beam (20) from the master hologram copy (6) byexposing the master hologram copy (6) to a second reference beam (10),and passing the readout beam (20) onto said decoding hologram (14). 2.The method as claimed in claim 1, wherein the readout beam (20) ispassed to said decoding hologram (14) through a fourier lens (12) in afourier plane from the master hologram copy (6).
 3. The method asclaimed in claim 1, comprising the step of rotating the hologram copy(6) to different degrees during examination.
 4. The method of claim 1,comprising the additional step ofgenerating and directing a referencebeam (10) onto said hologram copy (6) to generate the readout beam (20)therefrom.
 5. The method of claim 4, wherein said reference beam (10) isgenerated from a laser light source and the image-forming optical system(17) comprises a laser light source.
 6. The method of claim 5,comprising the additional step ofpositioning a fourier lens (12) as partof said optical arrangement in a fourier plane from the hologram copy(6), whereby it is possible to make adjustment in X and Y directionswithout adversely affecting hologram reconstruction, while, at the sametime, require insertion of the hologram copy (6) in a correct rotationalsetting for authentication.
 7. The method of claim 6, wherein saiddecoding hologram (14) possesses tolerances and unique features of theindividual reading device, such that, at most, only unique features ofthe decoding hologram (14) in the reading device could be copied, butnot features of the decoding hologram (14) in other reading devices,making it virtually impossible to reconstruct a master hologram fromsaid decoding hologram (14).
 8. The method of claim 7, comprising theadditional steps ofrecording the hologram copy (6) which is imitationproof by positioning photomaterial (6) in a first position on one sideof the fourier lens (3), positioning a statistical phase plate (1)containing a random distribution of phase elements on an opposite sideof the fourier lens(3), directing a distorted reference beam (5) at saidphotomaterial (6), and directing an object beam (2) from a laser lightsource through said phase plate (1) and fourier lens (3), causinginterference with said distorted reference beam (5) upon saidphotomaterial (6) and thereby recording said master program with phaseinformation.
 9. A reading device for examination of the authenticity ofhologram copies (6) of a master hologram, comprisingmeans for arrangingthe hologram copy (6) the authenticity of which is to be examined, in afirst position (11) of an optical arrangement, means for arranging adecoding hologram (14) in a second position (13) of said opticalarrangement, means for exposing said decoding hologram (14) with areadout beam (15) from an image-forming optical system (17), and meansfor checking authenticity of said hologram copy (6) by passing an object(readout) beam (20) therefrom onto said decoding hologram (14).
 10. Areading device as claimed in claim 9,whereinsaid optical arrangementcomprises a laser.
 11. A reading device as claimed in claim10,whereinsaid optical arrangement comprises a laser diode.
 12. Thereading device as claimed in claim 9, wherein said optical arrangementcomprises a fourier lens (12) in a fourier plane from said masterhologram copy (6).
 13. The reading device as claimed in claim 10,wherein said optical arrangement comprises a fourier lens (12) in afourier plane from said master hologram copy (6).
 14. The reading deviceas claimed in claim 9, comprising means for rotating said hologram copy(6) to different degrees during examination.
 15. The reading device asclaimed in claim 9, additionally comprisingmeans for generating anddirecting a reference beam (10) onto said hologram copy (6) to generatethe object beam (20) therefrom.
 16. The reading device as claimed inclaim 15, wherein said generating/directing means (10) and image-formingoptical management both comprise laser light sources.
 17. The readingdevice as claimed in claim 16, wherein said optical arrangementcomprises a fourier lens (12) positioned in a fourier plane from saidhologram copy (6), making it possible to make adjustment in X and Ydirections without adversely affecting hologram reconstruction, while atthe same time requiring insertion of the hologram copy (6) in a correctrotational setting for authentication.
 18. The reading device as claimedin claim 17, wherein said decoding hologram (14) possesses tolerancesand unique features of the individual reading device, such that, atmost, only unique features of the decoding hologram (14) in the readingdevice could be copied, but not features of the decoding hologram (14)in other reading devices, making it virtually impossible to reconstructa master hologram from said decoding hologram (14).
 19. The readingdevice as claimed in claim 18, wherein said reading device is structuredand arranged to be converted to apparatus for producing imitation-proofcopies (6) of a master hologram by positioning photomaterial (6) forcopying the master hologram in the position of the decoding hologram,and additionally comprisinga statistical phase plate (1) containing arandom distribution of phase elements on a side of said fourier lens (3)opposite said photomaterial (6), means for directing a distortedreference beam (5) at said photomaterial (6), and means for directing anobject beam (1) from a laser light source through said phase plate,(1),and fourier lens (3), to cause interference with said distortedreference beam (5) upon said photomaterial (6) and thereby record saidmaster hologram with phase information.